Early detection of disease is generally associated with improved health outcomes. For example, in emphysema the Global initiative for Chronic Obstructive Lung Disease (GOLD) staging is a well-accepted approach for categorizing disease as mild, moderate, severe, or very severe (Pauwels et al. 2001). However, GOLD scores are global with no regional information. In addition, they are based only on spirometry and clinical presentation. Thus, GOLD is a helpful non-invasive starting point, but uncertainty about its diagnosis may warrant additional diagnostic procedures such as CT (computed tomography) imaging (Pauwels et al. 2001). Furthermore many diseases, such as emphysema, are heterogeneous diseases, and signatures of tissue heterogeneity may facilitate more accurate diagnoses. An image-based metric would be useful for detecting such heterogeneities.